1. Field of the Invention
The present invention relates to a novel episulfide compound. More specifically, the present invention relates to a novel episulfide compound which can suitably be used as an optical material for plastic lenses, prisms, optical fibers, information recording substrates, filters and the like, above all, a material for plastic spectacle lenses.
2. Description of the Related Art
Plastic materials are lightweight, tough and easily dyeable, and for this reason, they have often been used as various optical materials, particularly spectacle lenses in recent years. As the performance of the optical materials, particularly the spectacle lenses, they are required to possess a low specific gravity, optical performances such as a high refractive index and a high Abbe's number, and physical performances such as a high heat resistance and a high strength. The high refractive index permits the lenses to be thin, the high Abbe's number decreases the chromatic aberration of the lenses, and the high heat resistance and the high strength can facilitate the secondary processing of the lenses and they are also important from the viewpoints of safety and the like. Typical plastic materials at an early stage of conventional techniques in this field are materials obtained by polymerizing compounds such as diethylene glycol bisallyl carbonate, a combination of this bisallyl carbonate and diallyl phthalate, and various kinds of methacrylates. These plastic materials have a refractive index of about 1.5 to 1.55, so that the obtained lenses are thick, and in consequence, the lightweight properties are lost. Therefore, the materials having the high refractive index have been desired, and various investigations have been conducted with the intention of obtaining a refractive index of 1.6 or more. There have already been suggested a polymer of a methacrylate compound containing a chlorine atom or a bromine atom, and a thermosetting optical material having a urethane structure obtained by the reaction of a hydroxy compound containing the bromine atom with an isocyanate (Japanese Patent Application Laid-open No. 164615/1983 and the like). However, when the compound containing the chlorine atom or the bromine atom is used, the specific gravity of the obtained lenses is large, and also in this case, the lightweight properties are eventually lost. Thus, thermosetting optical materials having thiourethane structures obtained by the reaction of polythiol compounds with polyisocyanate compounds have been suggested in Japanese Patent Publication No. 58489/1992 and Japanese Patent Application Laid-open No. 148340/1993. The various novel polythiol compounds which can be used as the materials of these thiourethanes have also been suggested. That is to say, Japanese Patent Application Laid-open No. 148340/1993 has suggested a branched polythiol compound having 4 sulfur atoms in one molecule; Japanese Patent Application Laid-open No. 270859/1990 has suggested a branched polythiol compound having 5 sulfur atoms in one molecule; and Japanese Patent Application Laid-open No. 192250/1994 has suggested a polythiol compound having a dithiane ring structure in one molecule. Additionally, in Japanese Patent Laid-open No. 81320/1991, there has been suggested a process for preparing a lens material by the use of a compound obtained by converting, into an episulfide group, a part or all of the epoxy groups of each of epoxy compounds such as known amine epoxy resins, phenolic epoxy resins, alcoholic epoxy resins, unsaturated compounds-containing epoxy resins, glycidyl ester epoxy resins, urethane epoxy resins and alicyclic epoxy resins. The thiourethane resin lenses which can be obtained by the polythiol compounds and the polyisocyanate compounds can possess a refractive index as high as about 1.66. However, episulfide resin lenses which can be obtained from episulfide compounds derived from known epoxy resins have a refractive index of at most about 1.6. Anyway, the problems of further thinning and reducing the weight of lenses can be solved to some extent by these conventional sulfur-containing compounds, but needless to say, a further higher refractive index is desirable. On the other hand, another important performance required for the optical material is that the chromatic aberration is low. The higher the Abbe's number is, the lower this chromatic aberration is, and therefore a material having the high Abbe's number is desired. That is to say, the simultaneous achievement of a high refractive index and a high Abbe's number is also desired. However, the Abbe's number usually tends to decline with an increase in the refractive index, and in plastic materials obtained by using conventional diethylene glycol bisallyl carbonate, known episulfide compounds and conventional compounds such as the polythiol compounds and the polyisocyanate compounds as raw materials, the Abbe's number is in the range of about 50 to 55 in the case of a refractive index of 1.5 to 1.55, and it is about 40 in the case of a refractive index of 1.60 and it is at most about 32 in the case of a refractive index of 1.66. On the other hand, the improvement of the heat resistance has often been tried by the use of a polyfunctional compound and a crosslinking agent, but in general, for the expression of a high refractive index, the molecular weight of the material compound is increased, so that the crosslink density decreases. For the expression of a high Abbe's number, the alkyl group content is increased, so that the stiffness of molecules constituting the material compound deteriorates and a sufficient improvement effect has not been obtained yet.
In the conventional optical materials obtained from the episulfide compounds and the combinations of the polythiol compounds and the isocyanate compounds, the increase in the refractive index is limited, and this increase in the refractive index leads to the deterioration of the Abbe's number. Therefore, there has been a problem that the sufficiently high refractive index and Abbe's number cannot be balanced with each other. Furthermore, the improvement of the above-mentioned optical properties, i.e., the refractive index and the Abbe's number leads to the deterioration of the heat resistance, and therefore there has been a problem that while the sufficiently high refractive index and Abbe's number are balanced with each other, the excellent heat resistance cannot be obtained.